Automatic milking device and method of operation thereof

ABSTRACT

A milking cup is moved into position surrounding a cow&#39;s teat by power operated members controlled by an electrical circuit including a pair of spaced ultrasonic detectors for locating the teat. Each detector includes a transmitter and a receiver. The ultrasonic fields produced by the transmitters intersect one another at a position spaced from the transmitters. Conical reflecting surfaces are disposed adjacent the transmitters and receivers to reflect ultrasonic waves to obtain a detection field of predetermined dimensions and to determine the area within which the ultrasonic waves are reflected back to the receivers. The generatrix of the conical reflecting surfaces may be slightly concave or convex.

A primary aspect of the present invention relates to an ultrasonicdetector.

Ultrasonic detector equipment for automatic milking devices is describedin the European patent application 232.568 from the same applicant. Aproblem in this known system is the obtaining of a detection field thatis accurately determined with respect to height, while in the planedefined by the height the widest possible bundle has to be generated inorder to be able to search for, find and/or approach a cow's udderprovided with a teat.

An ultrasonic detector has for its object to improve upon the knownprior art. Further this ultrasonic detector solves the above problem.

A further aspect of the present invention relates to a method forsearching for a moving object.

Existing robot systems, such as published in EP-A-232568, EP-A-213660and EP-A-209202, for seeking, finding and/or following a (moving) objectforming part of a greater entity have been found to be incapable ofmeeting requirements in respect of reliability, speed and/orpossibilities for error correction.

This further aspect of the present invention has for its object toimprove upon the above mentioned prior art. Further it is an object ofthe present invention to obviate the above mentioned drawbacks.

By making use, of position and speed information, a more accuratedetermining of position is achieved and information is obtained relatedto the pattern of movement of the (moving) object; informationconcerning speed in the case of violent movements will also beavailable.

With the method according to the invention information is obtainedconcerning errors and/or other irregularities either in the measuringprocess or the control process or in the movements of the object.

A further aspect of the present invention relates to an ultrasonicsensor unit.

Ultrasonic sensor units are much used in industrial and agrarianenvironments as they are not very sensitive to dirt and are robust.

Ultrasonic sensor units for use in agriculture are described in theEuropean patent applications EP-A-O 213 660, EP-A-O 232 568 and EP-A-O270 165.

The above publications all describe sensors for use in automatic milkingdevices, and more specifically for seeking and following the udderand/or teat of a cow.

By moving the transducer relative to an ultrasonic mirror the area forscanning is varied, for example made larger or smaller. Thus, forexample in the case of a conical reflection surface, this conicalreflection surface can be moved down or upward so that a narrower orwider bundle results.

In preference, however, the sensor unit is employed so that it becomespossible to scan an area around the transducer and by moving the mirroronly through a determined angle or by stopping the rotating ultrasonicmirror, it is possible to scan a more or less restricted circle segment.In order to follow a particular teat, the reflection surface will oftenbe brought to a stop.

A further aspect of the present invention relates to an element forpositioning an animal.

Such an element is to be used especially in an automatic milking systemin which it is important to position an animal in an exactly definedposition before automatically applying teat cups.

Another aspect of the present invention relates to a terminal apparatusto be used at an automatic milking system, in which a farmer can easilycontrol functions of the milking system.

Furthermore, yet another aspect of the present invention relates to amethod for automatically applying teat cups.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the present invention willbe clearified with reference to a drawing in which:

FIG. 1 shows a perspective view of an automatic milking device, in whichan ultrasonic detector according to the present invention is employed;

FIG. 2 shows a schematic, perspective view of detail II from FIG. 1;

FIG. 3 shows detail III from FIG. 2;

FIG. 4 shows a side view from the line IV--IV from FIG. 3; and

FIG. 5 shows detail V from FIG. 1.

FIG. 6 shows a perspective view of a robot system for implementation ofa part of a preferred embodiment of the method according to theinvention;

FIG. 7 shows a block diagram of the operation of the robot system fromFIG. 6;

FIG. 8 shows a block diagram of another preferred operation of the robotsystem from FIG. 7;

FIG. 9 shows a perspective, partly broken away and partly schematic viewof an automatic milking device provided with a preferred embodiment ofan ultrasonic sensor unit according to the present invention;

FIG. 10 shows the sensor unit, in more detail, beneath the udder of acow;

FIG. 11 shows another preferred embodiment of the ultrasonic sensor unitaccording to the present invention;

FIG. 12 shows a section along the line XI--XI from FIG. 11.

FIG. 13 shows a perspective view of an arrangement for automaticallyapplying teat cups to cows;

FIG. 14 shows a perspective view of detail XIV of FIG. 13;

FIG. 15 shows a perspective view of detail XV of FIG. 13;

FIG. 16 shows a perspective view of a terminal apparatus to be used at amilking system of FIG. 13;

FIG. 17 shows a top view of detail XVII of FIG. 16;

FIG. 18 shows in more detail detail XVIII of FIG. 16;

FIGS. 19A and 19B show respective perspective views of a detectionarrangement for automatically applying teat cups into respectivepositions; and

FIG. 20 a top view of a positioned cow.

DETAILED DESCRIPTION OF THE INVENTION

Mounted on the milking rack 2 are four milking cups 9, as described inEP-A-232.568. A gripper member 10 of the milking robot 1 grips onto themilking rack 2 and can move it three-dimensionally.

Arranged above the milking cups 9 is a sensor assembly 12 whichcomprises two (or more) ultrasonic transmitter/receiver units 13 and14--per se known--and arranged above them the respective cones 15 and 16(FIG. 2), preferably made of plastic. An example of signal processing ofsignals coming from ultrasonic sensors is described in EP-A-232.568.

Present in a housing 17 for the sensors is a space 18 for accommodationof the electronics, these electronics being connected in a manner notshown to a central control unit. The cones 15, 16 and the housing 17 arepreferably of aluminium or plastic--the cone surfaces polishedsmooth--so that the transmitter/receiver unit is robust, dirt-resistantand cannot rust.

If the transmitter/receiver units 13 and 14 are disposed at a mutualangle α of, for example, 50°, a detection field is created whereby oneteat can be located without the sensor units `hearing` one another'sdetection field; in order to avoid this cross over a screen 19 ofsound-damping material can be attached to housing 17.

It is also possible to use a transmitting member and two or morereceiving members at the same time. In this case it is also possible toscan two teats simultaneously.

In the preferred embodiment shown, the cone pieces 15, 16 are mounted onthe housing, for instance with adhesive.

Bundles of ultrasonic waves transmitted and received as according toarrows A by a transmitter/receiver unit 13 (FIG. 3, 4) form a broadbundle as indicated with the arrows C. The half top angle β of the coneamounts preferably to roughly 35°, while the cone is preferably disposedin slightly forward inclined position so that a generatrix G of the conesurface makes an angle α of approximately 45° with thetransmitter/receiver unit 13.

Through variation of the half top angle β of the cone and/or thedistance between the cone and the sensor, the area and/or shape of thefield can be adjusted.

The detection field will be rather flat and have the shape of aprojection of a cigar.

If there is a slight deviation from the cone shape, for instance ageneratrix G' has a slightly concave form, the height of the detectionfield--the distance between the bundle lines C--can thus be set. Ageneratrix G" may also have a slightly convex form.

By variation of the angle α, the principal direction of the field can beadjusted to the position of the sensor.

Via a piston rod 30 (FIG. 5), a top piece 31 of the milking cup 9 canautomatically be moved upward, while the position of a milking cuprelative to a teat 20' can be corrected using ultrasonictransmitter/receiver units 32 arranged close to at least one milkingcup. The ultrasonic waves are thereby directed against an ultrasonicmirror surface 33 to a point of intersection C' where the teat has to belocated in order to be able to place the top piece 31 around it in thecorrect manner. The mirror surface 33 consists, in preference, of aconical surface as shown in FIGS. 3 and 4, whereby a generatrix has aslightly convex form in order to obtain a bundle that is also broadenedin height.

The present invention is not limited to the above described embodiment,in which an integrated transmitter/receiver unit is applied. Thereflection surfaces can also be employed both in the case of a separatereceiver and a separate ultrasonic transmitter.

A further preferred embodiment relates to a method and robot system forthe milking of a cow. A robot system 41, comprising a number of armspivotable relative to each other, is movable along rails 47, 48 so as tobe capable of performing search, find and/or following operations indifferent locations for the arranging of milking cups 55 of a milkingrack 42 on a cow. The cow is directed with its head in the direction ofa feed bin 61 so that its udder will be situated in the vicinity of arest or start position of milking rack 42; feed bin 61 can be moved inaccordance with arrow B so that the udder of the cow will usually belocated in roughly the same or normal position relative to the milkingrack 42, independently of the length of the cow.

The cow will usually be recognized automatically and informationconcerning its udder and teats will be available, for instance, in acentral control means (not shown) provided with electronic hardwarehaving software stored therein. From an energizing member in cabinet 62,a mechanism 51 is driven (in a manner not shown) along upright rails 65,66 so that the rest position of mechanism 51, and therefore of milkingset 42, is pre-adjusted as far as possible subject to the size of thecow and the position of its udder. Further, movement in an upwarddirection is limited. Irrespective of the cow, the mechanism 51 is setalong the horizontal rails 63, 64 in a standard or fixed lengthwise restposition. Also driven from the electronics cabinet 62 is an actuator 46which controls, via a lever 45, two BOWDEN type cables which pull an armto which the milking rack 42 is attached into a predetermined rest orzero position.

Fixed to the side of milking rack 42 is a sensor assembly 52 for thepurpose of detecting, with the aid of two sensor units 53 and 54, amoving object, in the preferred embodiment shown a teat of a cow, inthree dimensions in a horizontal plane and passing on this informationto a central control means (not shown). Additional sensor units 69, 71can be disposed above each milking cup 55 of the milking set 42 in orderto keep a teat centred relative to a milking cup as the milking cupsmove upward. For the sake of clarity such fine sensor units 69, 71 areonly shown disposed by one milking cup 55. Like the sensor unit 52,these fine sensor units can be of an ultrasonic type.

A gripper member 50 of the robot 41 grips onto an opening of the milkingrack 42 arranged for that purpose. The robot mechanism 41 is capable ofmoving the milking rack 42 in three dimensions, while the mechanism 51can block changes of direction in a vertical direction; the electricaland program controls of the robot mechanism 41 and mechanism 51 arecoupled to one another in a manner not shown.

The operation of the robot system of FIG. 6 is explained on the basis ofthe diagram in FIG. 7 (and later on the basis of the diagram in FIG. 8).The position of the object, which is shown preferred embodiment as theposition of a teat of a cow, for example the teat at the frontright-hand side of the cow, with respect to the relevant sensor, formsthe input I for the sensors 52, 69, 71. The reflections measured by theultrasonic sensors 52, 69, 71 are converted in generally known mannerinto sampled digital values in a horizontal plane, in FIG. 7 indicatedwith Δx and Δy, these being relative position coordinates with respectto the sensors. Coming from the robot mechanism 41 are determined valuesx_(p) and y_(p), these being coordinates of the robot with respect tothe neutral position of the robot, whereby Δx and Δy--allowing forcorrection--must be added to them so that, as is indicated schematicallyin the figure, the value x_(m) and y_(m) result, these being thecoordinates of the teat with respect to the neutral position of therobot. These values are corrected in a manner described later so thatthe values x_(c) and y_(c) --the so-called targe coordinates--results,which can be fed to a block designated schematically with SC to give tothe robot 41 a new position towards which it must move the milking rack42.

The values x_(p) and y_(p), on the one hand, and Δx, Δy, on the other,are incorporated in a MODEL--which in the simplest case does not subjectthese said values to any processing whatever, i.e. it istransparent--whereby, after statistical analysis, comprising, forinstance, the determining of the mean or the standard deviation, anumber of M values are filtered out from a series of N values (M <N). Onthe basis of the (modulated) values a prediction (PRE) can be made whichis added to the controlling of the sensors in order to allow this PREvalue to take the place of clearly erroneous values from the series ofdigitalized measurement values, in accordance with a predeterminedcriterion. In the MODEL a Z-transformation replaces the digitalizedsamplings in the case of the described embodiment, but such a MODEL canlikewise be implemented with a so-called Kallman filter or in anothermanner. In a FPC filter, coefficients are determined in order toattribute a new value to the value x_(m) and y_(m) in PF on the basis ofthe MODEL as well as to the value v_(xm) --after differentiation in D ofthe values x_(m) and y_(m) --in order to add this filtered value(v_(xf), v_(yf)) after integration in (I) to the corrected filter valuex_(f) and y_(f).

The described control loop of Δx, Δy via x_(c), x_(y) to x_(s), y_(s) iseither closed or not on the basis of the data from the MODEL, as passedon to a block CONTROL 1. Only when predetermined criteria have beenfulfilled with respect to mean value of Δx over a determined time, meanvalue of Δy over a determined time, standard deviations therein inaddition to particular velocity values (v_(x), v_(y)), is the controlloop at S closed by a block designated FTL.

In the first instance the control loop at S remains opened since from ablock F, in a manner generally known in robot control, a globalpositional control is performed of the robot system 41 to a teat of acow using information stored in a memory. If the sensor unit 52--sensorunits 69, 71 are not used in these so-called find-modes--detects amoving object and predetermined criteria with respect to Δx, Δy, vx, vyand standard deviations thereof are satisfied, the control is taken overin a block T and the above criteria, which the above information mustsatisfy at an increased sampling speed, are made more stringent.

If the more stringent criteria are satisfied for a predetermined time,the control loop at S is closed and the values x_(c) and y_(c) are feddirectly to the servo-control (SC). If required the filter coefficientscomputed from the MODEL can be replaced by filter coefficients providedfrom the control means (CONTROL), these coefficients having a fixedvalue for a determined time duration, after which the coefficientcomputation from the MODEL will determine the position to which therobot arm is sent. Computations in CONTROL are dependent on the mode ofthe system.

If for a predetermined time duration the criteria from block T aresatisfied, the described control loop remains closed and block L takesover the controlling, whereby, as is indicated via line FS, it ispossible to switch over to fine sensor units 69, 71 so that the positionof the milking cup relative to the teat can be determined (still) moreaccurately using any required correction by the fine sensor units, andthe relevant milking cup 55 can be arranged if for a predetermined timethe teat reamins centred with respect to the milking cup and thereforethe sensor units.

After arrangement of the milking cup, a switch back is made to theT-mode until another teat of the cow is centred, following which thismilking cup can also be arranged in the L-mode.

It will be apparent that in the case of unexpected movements of the(moving) object the described robot system will change over instantlyfrom L-mode control to T-mode control or even to F-mode control.

A diagram of another preferred embodiment of the operation of the robotsystem of FIG. 6 (FIG. 8) requires no velocity filter and only thevalues Δx and Δy are hereby analyzed, while the MODEL forming isomitted. Prediction values are fed via block CONTROL to the sensor block52, 69, 71.

It is further noted that the Z coordinate or height coordinate of theteat of the cow will usually be constant and in the embodiment shownwill have a value that is fixed or determined by the CONTROL block. Inthe case of the embodiment shown in FIG. 8, the filter coefficients areconstant in a particular situation of the system (F-, T- or L-mode), butthey have a different value for each situation.

Extensive analyses and evaluation of tests have shown that with samplingat 20 Hz of the signals from the sensor assembly 52 and sensor units 69,71, and accurate model of the movements of a teat of a cow can be builtup. The controlling of the robot arm via the servo-control takes placein the preferred embodiment shown every 5 msec. In view of thepossibility of switching between different movement modes, this has beenfound in practice to be amply sufficient to be able to follow a teat ofa cow and arrange or attach the milking cups in a precise manner.

A robot installation 101 (FIG. 9) or similar installation for automaticarrangement of milking cups 102 on the udder of a cow and the subsequentremoval of milk is described in the above mentioned patent applicationsof the same applicant. The pivotable robot arm 103 is automaticallycontrollable using an ultrasonic sensor assembly 106 according to thepresent invention which is disposed centrally on a milking set 107.

Ultrasonic sensor assembly 106 (FIG. 10) comprises an ultrasonic sensorunit or transducer 108 which transmits and receives ultrasonic waves inaccordance with arrows D. Disposed above the ultrasonic transducer 108is an ultrasonic mirror 109. This ultrasonic mirror 109 is arranged in ahollow tube 111 which can be driven using a gear transmission 112 by aschematically indicated electric motor 113. A post or support 114 is infixed connection to the milking set 107 and provided with terminals 116and 117 for electrical connection of the transducer 108. The tube 111 isrotatable about post 114 via schematically indicated bearings 118, 119.

The electric motor can be driven electrically such that the tube 111 andtherefore the mirror 109 turn completely around and therefore a circularor control area around the transducer is scanned. This will be the caseif the mutual positions of teats 121 of the udder 122 of a cow have tobe determined. If an udder or teat of a cow has to be traced from aposition as shown in FIG. 9, the tube 111 can, for example, only beturned by the electric motor 113 back and forth through a limited angle,so that a limited control or circle segment is scanned, in which circlesegment the udder or teat can be expected to be because of the known,determined position of the cow.

In the position shown in FIG. 10 - a view from the hind side of thecow--the milking cup on the front righthand side of the milking set 107is carried or pushed upward and the tube 111 will become virtuallystationary relative to transducer 108 so that the teat 121 on the frontrighthand side of the udder 122 is continually detected by theultrasonic transducer 108. If a deviation is detected in the correctposition of the teat 121 relative to the cup 102, the milking set 107can usually be adjusted instantly as according to arrows E, thissubstantially two-dimensionally.

Another preferred embodiment of an ultrasonic sensor unit 126 (FIG. 11)according to the present invention is attached using a one-piece spring127 to a supporting construction 128 of a rack 129 for four milking cupswhich are omitted for the sake of clarity. Only the points ofattachement 131 for the milking cups to a tray 132 are shown. The tray132 is fixed for limited movement to the supporting construction 128with springs 133 and screw bolts 140. The sensor unit 126 protrudes fromthe side between the milking cups (not shown). Using socket head screws134 and slots 135, a housing 136, and with it an ultrasonic mirror 137,can be adjustably fixed in position relative to a connecting piece 138and therefore, the supporting construction 128. In addition an electricmotor 139 is fixed in position in the housing 136 with a screw bolt 141;a gear ring 143 engaging a driving gear wheel 142 and firmly joined tothe ultrasonic mirror 137 is thus coupled in housing 136 to the electricmotor 139. Further disposed in the housing is an upward facingtransducer 144 with a schematically indicated connection 146. Theultrasonic waves are transmitted according to the dashed and dotted lineF.

As can be seen in FIG. 12, the ultrasonic mirror may take a form that isslightly concave in the vertical direction, and in the horizontaldirection (FIG. 11) so that in addition to being reflected by the mirrorthe ultrasonic waves are also focused. If the ultrasonic bundle has todiverge, the ultrasonic mirror face can take a slightly convex form.

The electric motors 113 and 139 preferably take the form of so-calledstep motors so that the position and the angular rotation of theultrasonic mirror face are not measured or computed from the time ofrevolution, but the position is available directly at the output of thestep motor.

In accordance with the embodiments of the present invention bothdistance and direction of a (moving) object relative to the fixedtransducer are measured in simple manner.

In another preferred embodiment of an automatic milking system (FIG. 13)a moveable robot arm 201 is used to service milking locations 202 wherethe cow is retained in exact positions by means of positioning elements203 and moveable feeding containers 204.

A cow (FIG. 20) is forced to stand still on her right hind hoof by meansof a solid bar 261 for pushing the weight of the cow to its other hindhoof, such that an arm of a milking rack can be moved to the udder ofthe cow.

In more detail (FIG. 14) it is to be seen that on the center of themilking rack 207, a rotating scanner 208 is installed and a detectorunit 209 is disposed beside the milking rack.

A positioning element 203 (FIG. 15) comprises preferably retainingsurfaces 211 for retaining the hind hooves of a cow and inclidedsurfaces 212, 213 respectively to prevent the cow from moving her pawsfrom the retaining surfaces in sideward and forward directions,respectively. The hind part of the positioning element is provided withbars 214, 215 which will prevent the cow from moving her hoof in abackward direction and will allow excrement of the cow to fall through.The sidewardly inclined surfaces 212 extend to a height h, approximately70 mm above the ground level without providing a flat surface for thecow to stand on. This is however low enough to allow the robot arm to goto the udder of the cow. The length is approximately 300 mm viz.approximately two times the length of a hoof. Preferably the rear bar215 has a higher distance from the ground than the bars 214, such as toprevent the cow stepping backwardly.

An automatic milking system, such as shown in FIG. 13, is preferablycontrolled by means of a terminal control unit 221 (FIG. 16) providedwith a slot 222 for inserting a floppy disc, a screen 223 and aspecially designed keyboard 224.

The keyboard 224 is preferably provided with function keys such as 226,227 and 228. These are preferably in groups such as 229, 231 and 232which contain indicia thereon to indicate various functions. Key 226'will e.g. control, after pressing this key, the applying of teat cups toa cow.

As the cows will be provided with means for automatically recognizingthem, a complete history of the cows can be stored in a memory, e.g. ona floppy disc.

The keyboard 224 can easily be designed from a standard availablekeyboard, such as from a IBM or compatible computer. The farmer,however, will not be bothered by learning difficult codes, e.g. fromthree letters, by heart; as the function keys preferably are providedwith pectograms.

In the preferred embodiment of FIG. 13, using a positioning element 203and a feeding container adjustable relative to the length of the cow,the position of the udder and teat to be found are within a `window` ofapproximately 30×40 cm². Tests in this respect were done and even in thesituation that teat positions of cows were unknown to a computer beforeentering a milking location, the detection unit 209 (FIGS. 14, 19A, 19B)was able to pick up the position of a teat inside that `window`.

Preferably the detector unit 209 consists of transducer 210, 210' resp.provided above conical surfaces 235, 234 resp., such that no dirt willfall on the transducing surfaces.

The robot arm will move the milk rack 236 to the `window` or area inwhich the udder will be detected. Firstly the right teat T at the frontwill be searched, by moving the milk rack and sensor unit 209 up anddown. By analyzing the sample data from the sensor unit 209, it can beestablished that this right front teat T has been found.

The size of the `window` through which a sensor can `look` can bechanged dependent on the information received. Information from outsidethe `window` can be disregarded.

Afterwards a rotating scanner 237 will take over controlling theposition of the milking rack. This rotating scanner will then be underthe udder in between the teats. Under control of this rotating scanner237, the teat cups are connected to the teats, one at the time, asdescribed in the prior art.

When a teat cup is connected in the right way, this fact will be sensedby a sensor in a vacuum line of the milking system. Teat cups 238 arepreferably provided with flexible skirts 239 such as to prevent suckingair from the environment, which would disturb the ultrasonic detectorunits 209 and 237.

The sensor unit 237 is disposed to the milking rack through legs 241connected to bearings 242, 243 respectively, and, can be moved up anddown by means of an pneumatic cylinder 244.

A milking platform 246 as well as a connecting arm 247 are connected tothe frame 248 of the milking rack by means of springs 249, 251respectively. In the unlikely event that the cow will put her righthindleg on the milking rack, this milking rack will give away and willbe disconnected automatically.

In the preferred embodiment of FIG. 19A and 19B a reflecting surface237' of the unit 237 can be rotated at approximately 167 rpm. The sensorunit 237 further comprises encoding means 252 connected to the computer,an electric motor 253, gearing 254, bevel gear 256 and crown wheel 257.Under the rotating mirror 237', a transducing surface 258 is disposed,from which also detected reflections are transmitted to data processingequipment of the computerized controlling unit (not shown). Thetransducer uses a frequency of approx. 400 kH, such that it is notdisturbed by noise from the environment. Through the high revolutionspeed of the mirror, a focussing effect for the mirror is achieved.

We claim:
 1. An automatic milking device comprising a plurality ofmilking cups to be placed around the teats of a cow, moving means formoving said cups into operative position relative to the teats of a cow,and control means for controlling the movement of said moving means,said control means including ultrasonic detector means, said ultrasonicdetector means comprising a plurality of ultrasonic detectors spacedfrom one another, each of said ultrasonic detectors comprising atransmitter for transmitting ultrasonic waves and a receiver forreceiving ultrasonic waves, each of the transmitters producing anultrasonic detection field, the detection fields intersecting oneanother at a position spaced from said transmitters, and substantiallyconical reflection surface means disposed adjacent the transmitters andreceivers for reflecting ultrasonic waves transmitted by saidtransmitters to obtain a detection field of predetermined dimensions andfor reflecting ultrasonic waves received by said receivers to determinethe area within which the ultrasonic waves are reflected back to saidreceivers.
 2. A device as defined in claim 1 wherein the transmitter andreceiver of each of said ultrasonic detectors are integrated into atransmitter/receiver unit.
 3. A device as defined in claim 1 whereineach of the conical reflection surface means includes a center linedisposed at a slight angle with respect to vertical.
 4. A device asdefined in claim 1 wherein each of said conical reflection surface meansincludes a half top angle, each of said half top angles being betweenabout 20° and 45°.
 5. A device as defined in claim 1 wherein each ofsaid conical reflection surface means includes a conical surface, themagnitude of the angle between the generatrix of the conical surface anda line at a right angle to a vertical dissecting plane is approximately45°.
 6. A device as defined in claim 1 wherein each of said conicalreflection surface means includes a center line and a conical surface, ageneratrix of the conical surface being slightly concave with respect tothe center line of the associated conical reflection surface means.
 7. Adevice as defined in claim 1 wherein each of said conical reflectionsurface means includes a center line and a conical surface, a generatrixof the conical surface being slightly convex with respect to the centerline of the associated conical reflection surface means.
 8. A device asdefined in claim 1 wherein the plurality of ultrasonic detectors areoriented with one another so as to define an angle of about 50°therebetween.
 9. A device as defined in claim 8 including sound-dampingmeans disposed between said ultrasonic detectors to prevent one detectorfrom detecting the field of another detector.
 10. A device as defined inclaim 1 including sound-damping means disposed between said ultrasonicdetectors to prevent one detector from detecting the field of anotherdetector.